Homogeneous magnetic field generating structure



P. MEYERER Sept. 28, 1965 HOMOGENEOUS MAGNETIC FIELD GENERATING STRUCTURE Filed Dec. 12, 1960 United States Patent 3,209,199 HOMOGENEOUS MAGNETIC FIELD GENERATING STRUCTURE Paul Meyerer, Munich, Germany, assignor to Siemens &

Halske Aktiengesellschaft, Berlin and Munich, a corporation of Germany Filed Dec. 12, 1960, Ser. No. 75,197 Claims priority, application Germany, Mar. 10, 1960, S 67,508 2 Claims. (Cl. 315-535) This invention is concerned with an electron beam producing system for travelling wave tubes, comprising a metal cylinder which is disposed between acceleration anode and homogeneous magnetic field and surrounds the electron beam.

Flat or cylindrical electron beams are in known manner frequently produced by means of a so-called Pierce gun, such beams being guided along an extended path by means of a magnetic field. The operation of a return wave oscillator for millimeter waves requies, for exampe, electron beams With a diameter from to 7 millimeter and beam currents fom 10 to 50 milliamperes. The frequency of such an oscillator is determined by the voltage of the delay line. The tuning-through range in the frequency amounts to about one octave, requiring alteration of the voltage by a factor to 6. Voltage conditions demanding respectively from 600 volts to 3600 volts or from 200 volts to 1200 volts are thereby customary.

It is considered electron-optically, with known electrode arrangements in view of such great voltage alterations, impossible to guide the electron beam in the magnetic field without producing excessive waviness. The beam waviness may be kept considerably lower than in the case of pure deceleration when operating with a low anode voltage, that is, with successive acceleration. However, the perveances will thereby be in the order of magnitude of However, the disadvantage of electron guns with such great perveances, which are to produce a very fine electron beam with high current density, resides in small cathode-anode spacing, making these guns more sensitive to mechanical tolerances than guns with small perveance.

It is also known that electron guns with these perveances and dimensions, do not satisfy the theoretical assumption of homogeneous beam current density, which is due to thermal transverse velocities of the electrons and unavoidable lens errors in the anode.

There is an electron beam producing system known, having between the acceleration anode and the homogeneous magnetic field a second cylindrical anode which is at a potential higher than that of the acceleration anode, especially 1.1 to 1.5 times higher. This cylindrical anode straightens the electrons of the beam to some extent in parallel sense.

The object of the invention is to provide an electron beam producing system which is adapted to discharge the electron beam into the homogeneous magnetic field so that it extends therein without particular Waviness, thus mak- 3,209,199 Patented Sept. 28, .1965

ing it possible to construct the electron gun with optimum dimensions. In accordance with the invention, it is in connection with an electron beam producing system for travelling wave tubes having a metallic cylinder disposed between the acceleration anode and the homogeneous magnetic field and surrounding the electron beam, proposed to make .the length of the perveance cylinder at least equal to the fluctuating wave length of the electron beam and to make the potential of the perveance cylinder lower than that of the acceleration anode.

The invention will now be explained more in detail with reference to the accompanying drawing, in which FIG. 1 shows an example, in schematic representation, of an electron beam producing system for a return wave generator; and

FIG. 2 is a diagram showing the potential course ob taining between the cathode and the delay line.

Numeral 1 indicates the cathode and 2 the delay line of the return wave generator. A Wehnelt cylinder 3 and an acceleration anode 4 are provided in known manner. Between the acceleration anode 4 and the delay line 2 is disposed a metallic cylinder 5.

The axial magnetic guiding field must extend over the cylinder 5. It is thereby not required that the magnetic field course be homogeneous. The homogeneous magnetic field may, for example, start at the delay line 2 and the increase thereof may take place along the cylinder 5. Neither is it necessary that the magnetic field course be spatially uniform; it can also be periodic.

The cylindrical part 5 effects a perveance transformationsuch part being therefore designated as perveance cylinder-which, taken together with the axial magnetic field, influences the shape of the electron beam.

The length of the perveance cylinder must be at least equal to the fluctuating wave length of the electron beam. In the example shown in the drawing, the length of the perveance cylinder is appreciably greater than the fluctuating wave length A. The length of the perveance cylinder may be calculated according to the following formula:

whereby U is measured in volt and B in gauss.

This formula is valid only if the magnetic field strength is constant within the perveance cylinder. If the field strength is non-homogeneous, which may be the case when the increase of the magnetic field extends beyond the perveance cylinder, z must be greater or the perveance cylinder voltage Uperv must be lower. The voltage of the perveance cylinder shall be, if possible, as low as permissible, so that as many as possible fluctuating wave lengths of the electron beam appear within the perveance cylinder. This will make it possible to regulate within wide limits, by the variation of the voltage on the perveance cylinder, the entry conditions of the electron beam, such as entry radius and entry angle, into the homogeneous magnetic field.

For an optimum dimension of the electron gun, the anode voltage shall be high, so as to obtain a low perveance. For obtaining an electron gun with nearly constant radiation current density, the ratio of the cathode radius r;; to the radius of the anode curvature r shall likewise be equal or smaller than 2.

Changes may be made within the scope and spirit of the appended claims which define what is believed to be new and desired to have protected by Letters Patent.

I claim:

1. An electron beam producing system for travelling wave tubes having a cathode, an acceleration anode, and an interaction space which neighbors on the electron beam generating system, comprising a metal cylinder arranged between the acceleration anode and said interaction space, means for producing an axial magnetic field extending within the metal cylinder over the length thereof for action on the electron beam, the potential of said metal cylinder being lower than that of the acceleration anode with respect to the cathode potential, and the length of said metal cylinder being at least equal to the fluctuating wave length of the electron beam.

2. An electron beam producing system as defined in claim 1, wherein said magnetic field is homogeneous and the length of the metal cylinder, in centimeters, is greater with U being measured in volts and B in gauss.

References Cited by the Examiner UNITED STATES PATENTS 5/59 Brewer. 9/59 Knechtli.

GEORGE N. WESTBY, Primary Examiner.

RALPH G. NILSON, Examiner. 

1. AN ELECTRON BEAM PRODUCING SYSTEM FOR TRAVELLING WAVE TUBES HAVING A CATHODE, AND ACCELERATION ANODE, AND AN INTERACTION SPACE WHICH NEIGHBORS ON THE ELECTRON ARRANGED BETWEEN THE ACCELERATION ANODE AND SAID INTERACTION SPACE, MEANS FOR ACCELERATION ANODE AND SAID INTERACTION SPACE, MEANS FOR PRODUCING AN AXIAL MAGNETIC FIELD EXTENDING WITHIN THE METAL CYLINDER OVER THE LENGTH THEREOF FOR ACTION ON THE ELECTRON BEAM, THE POTENTIAL OF SAID METAL CYLINDER BEING LOWER THAT OF THE ACCELERATION ANODE WITH RESPECT TO THE CATHODE POTENTIAL, AND THE LENGTH OF SAID METAL CYLINDER BEING AT LEAST EQUAL TO THE FLUCTUATING WAVE LENGTH OF THE ELECTRON BEAM. 